Transgenic nonhuman primates would offer extremely accurate models of human disease. Transgenic mice offer powerful models for understanding gene function in the intact animal, but fundamental differences between mouse and human development, anatomy, and physiology limit the utility of transgenic mice for understanding many specific human diseases. Mouse embryonic stem (ES) cells are pluripotent cells that provide a vehicle for introducing very specific genetic changes into the mouse genome. We have recently reported the isolation of the first primate ES cell lines from the rhesus monkey. However, the reproductive characteristics of the rhesus monkey makes the use of rhesus ES cells to generate transgenic monkeys impractical. Common marmosets are primate species with more favorable reproductive characteristics for experimental embryology, including a short generation time, the routine birth of twins and triplets, and a prostaglandin sensitive corpus luteum that allows synchronization of reproductive cycles and efficient embryo transfer. In the proposal, we will: 1. Define the developmental potential of marmoset ES cells in embryoid bodies in vitro. 2. Determine the potential of marmoset ES cells to participate in normal development in chimeras in vitro and in vivo, and test whether the developmental potential of marmoset ES cells is stable or is decreased with continuous culture. 3. Test whether nuclei transferred from early passage ICM cells or marmoset ES cells to enucleated oocytes will support normal embryonic development. An accurate understanding of the developmental potential of marmoset ES cells will validate this new in vitro model for understanding the differentiation of human tissues, and will initiate exciting advances in experimental primate embryology. The ability to study the differentiation of specific primate lineages in vitro will provide an alternative for some developmental studies to using intact primate embryos recovered by cesarean section. Determining the frequency of germ line transmission by marmoset ES cells in chimeras is a prerequisite to generating transgenic marmoset models of human disease. If ES cell nuclei transferred to enucleated oocytes support complete development, then it will be possible to generate groups of genetically identical marmosets, which would greatly reduce the number of animals required to reach statistical significance in controlled experiments.